What can enemies see?
 

I suspect enemies can see through walls - is this true?

If it is, is there anyway to fix this (a la commandos where the cone of visionwould slice off whenever it hit an obstacle)? The reason I think they can see through walls is that on the first american mission I was at the arsenal in the shed with the bike and truck. After a few minutes checking out the crates i was 'spotted' by guards at the main gate even though there was a wall and a house between us. Also, why do guards sometimes also come running when you kill people silently?

Try clicking on the enemy unit and it will emit a red cone which is its line of site.
They may be seeing through a window, or they might hear them scream.